The present invention relates to a carboxylated nitrile group-containing highly saturated copolymer rubber, and a production process thereof, and more particularly to a carboxylated nitrile group-containing highly saturated copolymer rubber which is well balanced between abrasion resistance and strength properties and hard to undergo scorching, and a production process thereof.
A nitrile group-containing highly saturated copolymer rubber obtained by hydrogenating carbon-carbon double bonds in acrylonitrile-butadiene copolymer rubber to saturate it to a high degree is known as a rubber balanced among oil resistance, heat resistance, weather resistance, cold resistance and mechanical strength, and is used in, for example, application fields of belts and hoses. In recent years, such rubber has been required to improve the strength properties at both ordinary temperature and high temperature thereof to a higher level even in such application fields from the viewpoint of long-term use. In the application field of, for example, belts, it has been proposed to use a nitrile group-containing highly saturated copolymer rubber with maleic anhydride added thereto in order to more ensure the transmission of power (Japanese Patent Application Laid-Open No. 6039/1989).
Examples of a process for adding maleic anhydride to the nitrile group-containing highly saturated copolymer rubber include (1) a process in which an addition reaction is conducted in a solution state in a chemical reactor such as an autoclave, (2) a process in which a thermal addition reaction is conducted in a solid phase state using a mixing and processing machine such as a kneading extruder, and (3) a process in which a radical-type addition reaction is conducted in an internal kneader using a peroxide. However, the process (1) is expensive in production cost and moreover low in productivity. The process (2) involves problems of formation of burned matter, gel formation, mixing of the formed gel into a product, etc., and moreover tends to cause clogging of a die head due to gelling of a holdup at the outlet of the extruder, and so involves a problem of continuous productivity. In addition, the process (2) is difficult to remove unreacted maleic anhydride which remains in the rubber modified. The process (3) tends to cause gel formation.
As described above, there has not been yet found under the circumstances any production process which can industrially satisfy as to the process for adding maleic anhydride to the nitrile group-containing highly saturated copolymer rubber.
Further, the maleic anhydride-added nitrile group-containing highly saturated copolymer rubbers (i.e., typical examples of xe2x80x9ccarboxylated nitrile group-containing highly saturated copolymer rubberxe2x80x9d) obtained by these conventional processes have involved a problem that they are poor in winding tendency on a roll and low in processability. Such rubber is liable to undergo scorching due to its high reaction rate to a crosslinking aid such as zinc oxide, and so its molding or forming and processing ability is poor. The rubber has not sufficiently satisfied physical properties such as abrasion resistance and tensile strength which are required for belts and hoses.
It is an object of the present invention to provide a carboxylated nitrile group-containing highly saturated copolymer rubber which is excellent in tensile strength and abrasion resistance and high in adhesive strength to fiber, and a production process thereof.
Another object of the present invention is to provide a carboxylated nitrile group-containing highly saturated copolymer rubber which is hard to undergo scorching and excellent in processability into a roll, and a production process thereof.
A further object of the present invention is to provide a rubber composition comprising, as essential components, the carboxylated nitrile group-containing highly saturated copolymer rubber having such excellent various properties and a crosslinking agent.
A still further object of the present invention is to provide a composite comprising the above rubber composition and fiber.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. As a result, it has been found that in the maleic anhydride-added nitrile group-containing highly saturated copolymer rubbers obtained by the conventional processes, almost all the maleic anhydride groups added change to carboxyl groups. The present inventors have inferred that such addition state of the maleic anhydride group causes such problems as described above.
Therefore, the present inventors have carried out a further investigation as to the addition states of carboxylic acid anhydride groups and carboxyl groups, and addition reaction mechanisms in reactions for carboxylating nitrile group-containing highly saturated copolymer rubbers with an ethylenically unsaturated dicarboxylic acid or an anhydride thereof such as maleic anhydride. As a result, it has been found that the above objects can be achieved by carboxylated nitrile group-containing highly saturated copolymer rubbers in which proportions of the carboxylic acid anhydride groups and carboxyl groups added fall within respective specific ranges.
The carboxylated nitrile group-containing highly saturated copolymer rubbers having such excellent various properties can be obtained by conducting the ene-type addition reaction of a nitrile group-containing highly saturated copolymer rubber (hydrogenated acrylonitrile-butadiene copolymer rubber) with an ethylenically unsaturated dicarboxylic acid or an anhydride thereof (for example, maleic anhydride) under specific reaction conditions. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a carboxylated nitrile group-containing copolymer rubber having the following physical properties:
(a) the acid equivalent being at least 1xc3x9710xe2x88x924 ephr,
(b) the ratio of [peak height of a carboxylic acid anhydride group]/([peak height of the carboxylic acid anhydride group]+[peak height of a carboxyl group]) in its infrared absorption analysis being at least 0.5,
(c) the Mooney viscosity being 15 to 200, and
(d) the iodine value being at most 80 grams per 100 grams.
According to the present invention, there is also provided a process for producing a carboxylated nitrile group-containing highly saturated copolymer rubber by additionally reacting a nitrile group-containing highly saturated copolymer rubber with an ethylenically unsaturated dicarboxylic acid or an anhydride thereof, the process comprising the steps of:
(1) feeding the nitrile group-containing highly saturated copolymer rubber to an internal hot kneader and heating and kneading the rubber to control the temperature of the rubber within a range of 60 to 170xc2x0 C.,
(2) feeding the ethylenically unsaturated dicarboxylic acid or the anhydride thereof to the kneader while the rubber temperature falls within the range of 60 to 170xc2x0 C. to continue the kneading, and
(3) controlling the rubber temperature within a range of 200 to 280xc2x0 C. to additionally react the nitrile group-containing highly saturated copolymer rubber with the ethylenically unsaturated dicarboxylic acid or the anhydride thereof.
According to the present invention, there is further provided a rubber composition comprising, as essential components,
(A) a carboxylated nitrile group-containing copolymer rubber having the following physical properties:
(a) the acid equivalent being at least 1xc3x9710xe2x88x924 ephr,
(b) the ratio of [peak height of a carboxylic acid anhydride group]/([peak height of the carboxylic acid anhydride group]+[peak height of a carboxyl group]) in its infrared absorption analysis being at least 0.5,
(c) the Mooney viscosity being 15 to 200, and
(d) the iodine value being at most 80 grams per 100 grams.
(B) a crosslinking agent.
According to the present invention, there is provided a composite comprising the above rubber composition and fiber combined with each other.